The present invention relates to a safety switch or a signalling device for a safety circuit arrangement, and more particularly to a safety switch that comprises a diagnostic function for identifying a malfunction or a functional fault.
EP 1 363 306 A2 discloses a safety switch for monitoring the position of protective doors, machine covering parts or other protective devices for protecting rotating machine parts or other hazardous places. Such a safety switch is sometimes referred to as a signalling device. The safety switch is used for generating a safety-related signal which is supplied to a higher-level safety controller. The safety controller is configured to evaluate the signals of a number of safety switches or signalling devices and, in dependence thereon, to bring the hazardous installation into a safe state, for example switching off moving drive systems. In this arrangement, the safety controller is generally implemented separately from the operational controller of the installation, since the safety roller must meet much higher standards of fail-safety compared to a “simple” operational controller.
However, safety switches in the sense of the present invention are not only signalling devices but also what is typically called safety switching devices or safety relays and even programmable safety controllers, i.e. even those devices which are typically designed to evaluate input signals from signalling devices. Nevertheless, the preferred field of application of the present invention is signalling devices for safety-related applications. Such signalling devices are sometimes also called safety sensors.
Signalling devices/sensors for safety-related applications must be constructed to be fail safe, as the safety controllers/safety switching devices too, in order to ensure that the intended safety function is fulfilled at any time. Intelligent sensors, such as light barriers or light curtains, are therefore typically constructed to be inherently fail safe so that they meet categories 3 or even 4 of the European standard EN 954-1 or similar safety requirements. Although simple signalling devices such as emergency-off buttons, protective door switches, two-handed switches and the like are also redundant, as a rule, they do not have any, or have only a limited intelligence, in practice. As a rule, fault monitoring is then implemented by means of the higher-level safety controller or the higher-level safety switching device.
EP 1 363 306 A2, mentioned before, discloses a safety switch which has its own diagnostic function. In particular, tis known safety switch is capable of monitoring the operation of the switching elements by reading back its output signals.
The document also discloses a safety circuit arrangement in which a number of such safety switches are connected in series with one another to a higher-level safety controller. In this arrangement, an enable signal is looped through the switching elements of the individual safety switches up to the higher-level safety controller. As soon as one of the safety switches suppresses the enable signal by means of its switching elements, the enable signal is missing at the safety controller which thereupon places the monitored installation into a safe position of rest.
However, with the known safety circuit, the individual safety switches (signalling devices) cannot inform the safety controller about the results of their diagnosis. Although a fast switch-off of the monitored installation is thus enabled, it is not possible to draw a conclusion about the cause of the switch-off.
In more intelligent sensors such as, for example, light barriers or light curtains, it is known to signal diagnostic information to a higher-level safety controller by using separate diagnostic lines or a field bus connection for this purpose. This would be too complex and too expensive for “small” signalling devices, particularly mechanically operated signalling devices.